Method and an apparatus for heating glass tubes

Abstract

A method of heating glass tubes, particularly quartz tubes for the manufacture of optical fibres, where the tube is pre-heated, preferably by means of a gas flame, whereafter the tube is heated by means of microwave energy generated by a microwave generator (1), by introducing the tube axially into a microwave cavity (3) comprising in its end walls (6,7) or end surfaces openings for the tube. According to the invention, at least one electrical field is given a field image according to a TM-0n0-mode, preferably according to the TM-020-mode, which mode has a maxima (10) along the central axis (9;c-c) of the cavity (3) and a minima (11,12) on each side of and radially spaced from the central axis (9;c-c), where the electrical field portion located between the minima (11,12) is used for heating the wall of the glass tube (8). According to a preferred embodiment, an additional electrical field is caused to be formed by means of a second microwave generator (15), which field is given a field image according to a TE-01n-mode, preferably according to the mode TE-011, which field therewith is superposed upon the field according to the TM-0n0-mode.

I claim: 
     
       1. A method of heating glass tubes, particularly quartz tubes for the manufacture of optical fibres, by preheating the tube, preferably by means of a gas flame, whereafter the tube is heated by means of microwave energy generated by a microwave generator (1), by introducing the tube axially into a microwave cavity (3) comprising in its end walls (6,7) or end surfaces openings for the tube, characterized in that two electrical fields according to different modes are caused to be formed in said cavity, in that a first of said electrical fields is caused to be formed by a first microwave generator (1) and the second of said electrical fields by a second microwave generator (15) in that the first electrical field is given a field image according to a TM-0n0-mode, preferably according to the TM-020-mode, which mode has a maxima (10) along the central axis (9;c--c) of the cavity and a minima (11,12) on each side of a radially spaced from the central axis (9;c--c), where the electrical field portion located between said minima (11,12) is utilized for heating the wall of said glass tube (8), and in that the second electrical field is given a field image according to a TE-01n-mode, preferably according to the mode TE-011, which field therewith is simultaneous with the superposed upon the field according to the TM-0n0-mode. 
     
     
       2. A method as defined in claim 1, characterized in that at least said firstmentioned microwave generator (1) is caused to generate said electrical fields during a deposition and, respectively, sintering phase, where layers with different substances are applied on the inside of the glass tube, and that during a subsequent collapsing phase, where the tube is to be collapsed to a rod, the said two microwave generators (1;15) are caused to generate each its respective electrical field according to the modes TM-0n0 and, respectively, TE-01n. 
     
     
       3. A method as defined in claim 1, characterized in that a microwave cavity (3) of metal is used, which is axial-symmetrical in respect of an axis (9) extending through the centre of said openings (4,5). 
     
     
       4. An apparatus for heating glass tubes, particularly quartz tubes for the manufacture of optical fibres, which apparatus comprises means for pre-heating the glass tube, preferably a gas flame, and comprises microwave generators connected to a microwave cavity, which is provided with openings in its end walls or end surfaces for passing a glass tube through the cavity, characterized in that a first and a second microwave generator (1;15) via a respective wave guide (2;16) are connected to the cavity (3), so that a first electrical field from one of said generators is given a field image according to a TM-0n0-mode, preferably according to the TM-020-mode, which mode has a maxima (10) along the central axis (9;c--c) of the cavity (3) and a minima (11,12) on each side of and radially spaced from the central axis (9;c--c) and so that a second electrical field from the other of said generators is given a field image according to a TE-01n-mode, preferably according to the mode TE-011, which second field therewith is simultaneous with and superposed upon the field according to the TM-0n0-mode, and in that a longitudinal axis through the centre of said openings (4,5) coincides with said central axis (9). 
     
     
       5. An apparatus as defined in claim 4, characterized in that a control device (17) is capable to connect and disconnect each of the said two microwave generators (1,15). 
     
     
       6. An apparatus as defined in claim 4, characterized in that the microwave cavity (3) is of metal and axial-symmetrical in respect of said central axis (9).